1. Field of the Invention
The present invention generally relates to a semiconductor device including a circuit constituted by thin-film transistors (referred to as a “TFT” hereinafter), and a manufacturing method thereof. More specifically, the present invention is directed to such an apparatus typically known as a liquid crystal display apparatus (namely, liquid crystal module is mounted) or the like, and also to an electronic appliance which mounts thereon such a liquid crystal display or the like as a component, and is further directed to a manufacturing method thereof.
It should be understood that a term “semiconductor device” implies all of such apparatus, or devices operable by using semiconductor characteristics, and thus, all of electrooptical apparatus, semiconductor devices, and electronic appliances correspond to semiconductor devices in the specification of the present invention.
2. Description of Related Art
Very recently, a specific attention has been paid to such a technique capable of constituting a thin-film transistor (TFT), while employing a semiconductor thin film (thickness thereof being several nm to several hundreds nm) which is formed on a substrate having an insulating surface. Thin-film transistors are widely utilized in various electronic devices such as ICs and electrooptical apparatus. In particular, there are strong demands to rapidly develop these thin-film transistors as switching elements of image display apparatus.
Conventionally, liquid crystal display devices are known as image display apparatus. Since high precision images can be obtained, large numbers of active matrix type liquid crystal display devices are usually employed, as compared with passive type liquid crystal display devices. In an active matrix type liquid crystal display device, since pixel electrodes arranged in a matrix form are driven, a display pattern is formed on a display screen. More specifically, since a voltage is applied between a selected pixel electrode and a counter electrode corresponding to this selected pixel electrode, liquid crystal layers arranged between the selected pixel electrode and the counter electrode are optically modulated, so that this optical modulation may be recognized as a display pattern by a viewer.
While such active matrix type liquid crystal display devices are widely used in various fields, strong demands are requested for enlarging display screen sizes, and for realizing high precision, high aperture efficiencies, and high reliability. At the same time, improvements in productivity and also low cost are strongly required.